MYOSIN CATALYTIC DOMAIN FLEXIBILITY IN MGADP

Conventional EPR studies of muscle fibers labeled with a novel alpha-i odoketo spin label at Cys-707 of the myosin head revealed substantial internal domain reorganization on the addition of ADP to rigor fibers. The spin probes that are well-ordered in the rigor state become disor dered and form two distinct populations. These orientational changes d o not correspond to rotation of the myosin catalytic domain as a whole because other probes (maleimide and iodoacetamide nitroxides attached to the same Cys-707 of myosin head) report only a small (5-10 degrees ) torsional rotation and little or no change in the tilt angle [Ajtai et al. (1992) Biochemistry 31, 207-17; Fajer (1994) Biophys. J. 66, 20 39-50]. In the presence of ADP, the labeled domain becomes more flexib le and executes large-amplitude microsecond motions, as measured by sa turation-transfer EPR with rates (tau(r) = 150 mu s) intermediate betw een the rotations of detached (tau(r) = 7 mu s) and rigor heads (tau(r ) = 2500 mu s). This finding contrasts with an absence of global motio n of the myosin head in ADP (tau(r) = 2200 mu s) as reported by the ma leimide spin label. Our results imply that the myosin head in a single chemical state (AM.ADP) is capable of attaining many internal configu rations, some of which are dynamic. The presence of these slow structu ral fluctuations might be related to the slow release of the hydrolysi s products of actomyosin ATPase.